U.S. patent application number 16/480428 was filed with the patent office on 2020-01-09 for high-strength hot-dipped steel sheet having excellent coating adhesion and method for manufacturing same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Yusuke FUSHIWAKI, Masaki KOBA, Yasunobu NAGATAKI.
Application Number | 20200009833 16/480428 |
Document ID | / |
Family ID | 62978303 |
Filed Date | 2020-01-09 |
United States Patent
Application |
20200009833 |
Kind Code |
A1 |
KOBA; Masaki ; et
al. |
January 9, 2020 |
HIGH-STRENGTH HOT-DIPPED STEEL SHEET HAVING EXCELLENT COATING
ADHESION AND METHOD FOR MANUFACTURING SAME
Abstract
There is provided a high-strength hot-dipped steel sheet having
excellent coating adhesion and a method for manufacturing the steel
sheet. The steel sheet has a chemical composition, and a coating
layer is disposed on the steel sheet. The chemical composition
includes, by mass %, C: 0.02% or greater and 0.30% or less, Si:
0.01% or greater and 2.0% or less, Mn: 0.2% or greater and 3.0% or
less, P: 0.08% or less, S: 0.02% or less, and Al: 0.001% or greater
and 0.40% or less, with the balance being Fe and incidental
impurities. The coating layer has a coating weight per side of 30
to 90 g/m.sup.2 and contains exfoliated base steel in an amount of
0.3 to 1.5 g/m.sup.2.
Inventors: |
KOBA; Masaki; (Tokyo,
JP) ; FUSHIWAKI; Yusuke; (Tokyo, JP) ;
NAGATAKI; Yasunobu; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
62978303 |
Appl. No.: |
16/480428 |
Filed: |
January 10, 2018 |
PCT Filed: |
January 10, 2018 |
PCT NO: |
PCT/JP2018/000247 |
371 Date: |
July 24, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/0236 20130101;
C21D 6/008 20130101; C21D 1/76 20130101; C23C 2/40 20130101; C21D
8/0205 20130101; C22C 38/06 20130101; C22C 38/14 20130101; C21D
9/46 20130101; C22C 38/002 20130101; C23C 2/28 20130101; C21D
8/0273 20130101; C23C 2/02 20130101; C23C 2/06 20130101; C22C 38/04
20130101; C21D 8/0226 20130101; C21D 6/005 20130101; B32B 15/013
20130101; C21D 8/0263 20130101; C22C 38/02 20130101; C23C 2/12
20130101; C22C 38/00 20130101; C22C 38/001 20130101; C22C 38/12
20130101 |
International
Class: |
B32B 15/01 20060101
B32B015/01; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 9/46 20060101 C21D009/46; C21D 8/02 20060101
C21D008/02; C21D 6/00 20060101 C21D006/00; C23C 2/02 20060101
C23C002/02; C23C 2/28 20060101 C23C002/28; C23C 2/40 20060101
C23C002/40; C23C 2/06 20060101 C23C002/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2017 |
JP |
2017-010787 |
Claims
1. A high-strength hot-dipped steel sheet having a chemical
composition comprising, by mass %: C: 0.02% or greater and 0.30% or
less, Si: 0.01% or greater and 2.0% or less, Mn: 0.2% or greater
and 3.0% or less, P: 0.08% or less, S: 0.02% or less, and Al:
0.001% or greater and 0.40% or less, with a balance being Fe and
incidental impurities, the steel sheet comprising a coating layer
disposed thereon, wherein the coating layer has a coating weight
per side in a range of 30 to 90 g/m.sup.2 and contains exfoliated
base steel in an amount in a range of 0.3 to 1.5 g/m.sup.2.
2. The high-strength hot-dipped steel sheet having excellent
coating adhesion according to claim 1, wherein the chemical
composition further comprises, by mass %, at least one selected
from the group consisting of Ti: 0.01% or greater and 0.40% or
less, Nb: 0.001% or greater and 0.200% or less, V: 0.001% or
greater and 0.500% or less, Mo: 0.01% or greater and 0.50% or less,
W: 0.001% or greater and 0.200% or less, and B: 0.0003% or greater
and 0.01% or less.
3. A method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion, wherein a steel slab
having the chemical composition according to claim 1 is subjected
to hot rolling, rolling at a rolling reduction ratio in a range of
1 to 10%, and pickling, and subsequently, to rolling at a rolling
reduction ratio in a range of 0.3 to 5% and hot-dip coating.
4. The method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion according to claim 3,
wherein, in the hot rolling, finish rolling is performed after
rough rolling at a finish rolling temperature of 820.degree. C. or
higher, and subsequently, coiling is performed at a coiling
temperature in a range of 450 to 650.degree. C.
5. The method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion according to claim 3,
wherein, after the rolling at a rolling reduction ratio in the
range of 0.3 to 5% and prior to the hot-dip coating, continuous
annealing is performed in a furnace atmosphere having a hydrogen
concentration in a range of 2 to 30 vol % and a dew point in a
range of -60 to -10.degree. C., with a steel sheet annealing
end-point temperature being in a range of 600 to 950.degree. C.
6. The method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion according to claim 3,
wherein alloying is additionally performed after the hot-dip
coating.
7. The high-strength hot-dipped steel sheet having excellent
coating adhesion according to claim 3, wherein the chemical
composition further comprises, by mass %, at least one selected
from the group consisting of Ti: 0.01% or greater and 0.40% or
less, Nb: 0.001% or greater and 0.200% or less, V: 0.001% or
greater and 0.500% or less, Mo: 0.01% or greater and 0.50% or less,
W: 0.001% or greater and 0.200% or less, and B: 0.0003% or greater
and 0.01% or less.
8. The method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion according to claim 4,
wherein, after the rolling at a rolling reduction ratio in the
range of 0.3 to 5% and prior to the hot-dip coating, continuous
annealing is performed in a furnace atmosphere having a hydrogen
concentration in a range of 2 to 30 vol % and a dew point in a
range of -60 to -10.degree. C., with a steel sheet annealing
end-point temperature being in a range of 600 to 950.degree. C.
9. The method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion according to claim 4,
wherein alloying is additionally performed after the hot-dip
coating.
10. The method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion according to claim 5,
wherein alloying is additionally performed after the hot-dip
coating.
11. The method for manufacturing a high-strength hot-dipped steel
sheet having excellent coating adhesion according to claim 8,
wherein alloying is additionally performed after the hot-dip
coating.
Description
TECHNICAL FIELD
[0001] This application relates to a high-strength hot-dipped steel
sheet having excellent coating adhesion and to a method for
manufacturing the steel sheet. The base metal of the steel sheet is
a high-strength steel sheet containing Si and Mn.
BACKGROUND
[0002] To date, in the fields of automotive steel sheets and the
like, surface-treated steel sheets obtained by imparting corrosion
protection properties to a base steel sheet have been used, and
particularly, galvanized steel sheets and galvannealed steel
sheets, which have excellent corrosion protection properties, have
been used. In recent years, however, a thickness reduction and
strength increase of steel sheets has been promoted to improve fuel
economy and improve crashworthiness.
[0003] Typically, hot-dipped steel sheets are manufactured as
follows. A steel sheet obtained by hot-rolling a slab or further by
cold rolling is used as the base metal, and the base metal steel
sheet is subjected to recrystallization-annealing in an annealing
furnace of a CGL and thereafter subjected to hot-dip coating.
Furthermore, alloyed hot-dipped steel sheets are manufactured by
performing hot-dip coating and thereafter further performing
alloying.
[0004] For hot-dipped steel sheets that are used for applications
such as those described above, coating adhesion sufficient to
withstand severe processing, such as a hole expansion processing,
is very important, in addition to good surface appearance. However,
hot-dipped steel sheets containing Si and Mn, which are oxidizable
elements, tend to have defects such as bare spots and non-alloyed
portions, and moreover, have deteriorated coating adhesion, as a
result of formation of Si and Mn oxides in the surface of the steel
during recrystallization annealing.
[0005] Several proposals have been made to solve the problems
described above. For example, Patent Literature 1 proposes a method
for improving powdering resistance. In the method, a Ni coating is
applied to a steel sheet prior to heating, and subsequently,
hot-dip coating is performed.
[0006] Patent Literature 2 proposes a method for improving the
adhesion of a coating film. In the method, a base metal is
subjected to surface grinding, heated to 600.degree. C. or higher
in a reducing atmosphere, and cooled. The base metal is subjected
to hot-dip coating and then to alloying.
[0007] Patent Literature 3 proposes a method for manufacturing a
galvannealed steel sheet. In the method, a hot-rolled steel sheet
or an annealed cold-rolled steel sheet is subjected to light
reduction rolling at a rolling reduction ratio of 1.0 to 20% and
then to a low-temperature heat treatment in which the steel sheet
is held at 520 to 650.degree. C. for five seconds or more, and
thereafter, the steel sheet is immersed in a hot-dip galvanizing
bath containing, in mass %, Al in an amount of 0.01 to 0.18% and
then subjected to alloying.
[0008] The method proposed in Patent Literature 1, however,
requires a step of applying a Ni coating to the surface of a base
metal prior to heating. The method proposed in Patent Literature 2
requires a grinding process. As such, in both Patent Literature 1
and Patent Literature 2, a costly and laborious process is
required, and therefore, there has been a problem in that
productivity decreases.
[0009] Furthermore, with the method proposed in Patent Literature
3, coating adhesion at a high level that sufficiently corresponds
to high strength and processability that are required of current
high-strength steel sheets has not been achieved, and therefore the
method has not necessarily been beneficial for the corrosion
resistance of processed portions.
CITATION LIST
Patent Literature
[0010] PTL 1: Japanese Unexamined Patent Application Publication
No. 2010-196146
[0011] PTL 2: Japanese Unexamined Patent Application Publication
No. 10-81948
[0012] PTL 3: Japanese Unexamined Patent Application Publication
No. 2002-317257
SUMMARY
Technical Problem
[0013] The disclosed embodiments have been made in view of the
above circumstances, and an object of the disclosed embodiments is
to provide a high-strength hot-dipped steel sheet having excellent
coating adhesion and a method for manufacturing the steel
sheet.
Solution to Problem
[0014] The present inventors diligently performed studies to solve
the problems described above. As a result, it was found that a
high-strength hot-dipped steel sheet having excellent coating
adhesion can be obtained by including exfoliated base steel in the
coating layer.
[0015] The disclosed embodiments were made based on the above
findings, and the features include the following.
[1] A high-strength hot-dipped steel sheet having excellent coating
adhesion, the high-strength hot-dipped steel sheet including: a
steel sheet having a chemical composition, and a coating layer
disposed on the steel sheet, the chemical composition including, in
mass %, C: 0.02% or greater and 0.30% or less, Si: 0.01% or greater
and 2.0% or less, Mn: 0.2% or greater and 3.0% or less, P: 0.08% or
less, S: 0.02% or less, and Al: 0.001% or greater and 0.40% or
less, with the balance being Fe and incidental impurities, the
coating layer having a coating weight per side of 30 to 90
g/m.sup.2, wherein the coating layer contains exfoliated base steel
in an amount of 0.3 to 1.5 g/m.sup.2. [2] The high-strength
hot-dipped steel sheet having excellent coating adhesion according
to [1], wherein the chemical composition further includes, in mass
%, one or more of Ti: 0.01% or greater and 0.40% or less, Nb:
0.001% or greater and 0.200% or less, V: 0.001% or greater and
0.500% or less, Mo: 0.01% or greater and 0.50% or less, W: 0.001%
or greater and 0.200% or less, and B: 0.0003% or greater and 0.01%
or less. [3] A method for manufacturing a high-strength hot-dipped
steel sheet having excellent coating adhesion, wherein a steel slab
having a chemical composition according to [1] or [2] is subjected
to hot rolling, rolling at a rolling reduction ratio of 1 to 10%,
and pickling, and subsequently, to rolling at a rolling reduction
ratio of 0.3 to 5% and hot-dip coating. [4] The method for
manufacturing a high-strength hot-dipped steel sheet having
excellent coating adhesion according to [3], wherein, in the hot
rolling, finish rolling is performed after rough rolling, at a
finish rolling temperature of 820.degree. C. or higher, and
subsequently, coiling is performed at a coiling temperature of 450
to 650.degree. C. [5] The method for manufacturing a high-strength
hot-dipped steel sheet having excellent coating adhesion according
to [3] or [4], wherein, after the rolling at a rolling reduction
ratio of 0.3 to 5% and prior to the hot-dip coating, continuous
annealing is performed in a furnace atmosphere having a hydrogen
concentration of 2 to 30 vol % and a dew point of -60 to
-10.degree. C., with a steel sheet annealing end-point temperature
being 600 to 950.degree. C. [6] The method for manufacturing a
high-strength hot-dipped steel sheet having excellent coating
adhesion according to any one of [3] to [5], wherein alloying is
additionally performed after the hot-dip coating.
[0016] Note that the high-strength hot-dipped steel sheet of the
disclosed embodiments is a steel sheet having a tensile strength
(TS) of 590 MPa or greater and is obtained by performing hot-dip
coating or hot-dip coating plus subsequent alloying, on a base
metal, which is a hot-rolled steel sheet or a cold-rolled steel
sheet. Furthermore, examples of coatings herein include Zn
coatings, Zn--Al coatings, and Al coatings.
Advantageous Effects
[0017] The disclosed embodiments make it possible to obtain a
high-strength hot-dipped steel sheet having excellent coating
adhesion. Since the steel sheet exhibits high corrosion resistance
even after processing, the steel sheet is effective for
manufacturing a member having a complex shape, and, therefore, the
disclosed embodiments provide a significant industrial
advantage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is an image showing a state of exfoliated base steel
of a galvannealed steel sheet.
DETAILED DESCRIPTION
[0019] The disclosed embodiments will now be described in detail.
In the following descriptions, the contents of the elements in the
chemical composition of the steel and the contents of the elements
in the chemical composition of the coating are all in mass % but
are indicated simply with % unless otherwise specified. In
addition, the hydrogen concentrations are all in vol % but are
indicated simply with % unless otherwise specified.
[0020] According to the disclosed embodiments, a high-strength
hot-dipped steel sheet having excellent coating adhesion includes a
steel sheet and a coating layer disposed on the steel sheet. The
steel sheet includes, in mass %, C: 0.02% or greater and 0.30% or
less, Si: 0.01% or greater and 2.0% or less, Mn: 0.2% or greater
and 3.0% or less, P: 0.08% or less, S: 0.02% or less, and Al:
0.001% or greater and 0.40% or less, with the balance being Fe and
incidental impurities. The coating layer has a coating weight per
side of 30 to 90 g/m.sup.2. The coating layer contains exfoliated
base steel in an amount of 0.3 to 1.5 g/m.sup.2. That is, in the
disclosed embodiments, exfoliated base steel is incorporated into
the coating layer, thereby preventing propagation of cracks formed
in the coating layer during processing and thus improving coating
adhesion. As a result, the high-strength hot-dipped steel sheet
having excellent coating adhesion is obtained.
[0021] First, the following describes reasons for the limitations
on the chemical composition of the high-strength hot-dipped steel
sheet having excellent coating adhesion, to which the disclosed
embodiments are directed.
[0022] C: 0.02% or Greater and 0.30% or Less
[0023] The formability of a base metal improves as the C content
decreases, but the presence of C increases the strength of the
steel sheet inexpensively. Accordingly, the C content is greater
than or equal to 0.02%. It is preferable that the C content not be
less than 0.04%. On the other hand, the presence of an excessive
amount of C reduces the toughness and weldability of the steel
sheet, and therefore the C content is less than or equal to 0.30%.
It is preferable that the C content not be greater than 0.20%.
[0024] Si: 0.01% or Greater and 2.0% or Less
[0025] Si is effective as a solid solution strengthening element
and needs to be present in an amount of 0.01% or greater to
increase the strength of the steel sheet. However, the presence of
an excessive amount of Si degrades wettability for hot-dip coating
and degrades reactivity for alloying, which makes adjustment of
alloying difficult and results in a deterioration of coating
appearance and coating adhesion. For these reasons, the Si content
is 0.01% or greater and 2.0% or less.
[0026] Mn: 0.2% or Greater and 3.0% or Less
[0027] Mn is an element useful for increasing the strength of
steel. To produce this effect, Mn needs to be present in an amount
of 0.2% or greater. However, the presence of an excessive amount of
Mn degrades wettability for hot-dip coating and degrades reactivity
for alloying, which makes adjustment of alloying difficult and
results in a deterioration of coating appearance and coating
adhesion. For these reasons, the Mn content is 0.2% or greater and
3.0% or less. It is preferable that the Mn content be 0.3% or
greater and 2.6% or less.
[0028] P: 0.08% or Less
[0029] If P is present in an amount greater than 0.08%, weldability
deteriorates and surface quality deteriorates. Furthermore,
alloying needs to be performed at an increased alloying
temperature; otherwise, it is impossible to obtain a desired
alloying degree. However, if the alloying temperature is increased,
the ductility of the base metal steel sheet deteriorates and the
adhesion of the alloyed hot-dipped layer deteriorates. Accordingly,
the P content is less than or equal to 0.08%.
[0030] S: 0.02% or Less
[0031] Segregation of S at grain boundaries or formation of large
amounts of MnS reduces toughness, and therefore the S content needs
to be less than or equal to 0.02%. Accordingly, the S content is
less than or equal to 0.02%. The lower limit of the S content is
not particularly limited, and the content may be the level of that
of impurities.
[0032] Al: 0.001% or Greater and 0.40% or Less
[0033] Al is added for the purpose of deoxidation of molten steel.
If the content is less than 0.001%, the purpose is not achieved. On
the other hand, if Al is present in an amount greater than 0.40%,
large numbers of inclusions are formed, which causes a defect in
the steel sheet. Accordingly, the Al content is 0.001% or greater
and 0.40% or less.
[0034] The balance is Fe and incidental impurities.
[0035] In the disclosed embodiments, for the purpose described
below, one or more of the following may be further included, in
mass %, Ti: 0.01% or greater and 0.40% or less, Nb: 0.001% or
greater and 0.200% or less, V: 0.001% or greater and 0.500% or
less, Mo: 0.01% or greater and 0.50% or less, W: 0.001% or greater
and 0.200% or less, and B: 0.0003% or greater and 0.01% or
less.
[0036] Ti, Nb, V, Mo, W, and B are elements necessary to cause
precipitation of precipitates (carbides, in particular) in the base
metal steel sheet, and it is preferable to add one or more selected
from the group consisting of these elements. In general, in many
cases, these elements are present in the form of precipitates
containing these elements, in the base metal steel sheet. Of these
elements, Ti, in particular, is an element having a high
precipitation-strengthening ability and being effective from a cost
standpoint. If the amount of Ti added is less than 0.01%, however,
the amount of precipitates in the base metal steel sheet necessary
for ensuring that precipitates (carbides, in particular) are
included in the alloyed hot-dipped layer may be insufficient. On
the other hand, if the amount is greater than 0.40%, the effect no
longer increases and the cost increases. Accordingly, when Ti is to
be included, the Ti content is 0.01% or greater and 0.40% or
less.
Likewise, when Nb, V, Mo, W, and B are to be included, the contents
are as follows, for reasons similar to those for the upper limit
and the lower limit of the range of the Ti content. The Nb content
is 0.001% or greater and 0.200% or less, the V content is 0.001% or
greater and 0.500% or less, the Mo content is 0.01% or greater and
0.50% or less, the W content is 0.001% or greater and 0.200% or
less, and the B content is 0.0003% or greater and 0.01% or
less.
[0037] Next, the coating layer will be described.
The coating layer has a coating weight per side of 30 to 90
g/m.sup.2. If the coating weight is less than 30 g/m.sup.2,
ensuring corrosion resistance is difficult. On the other hand, if
the coating weight is greater than 90 g/m.sup.2, coating peel
resistance deteriorates.
[0038] Furthermore, exfoliated base steel is included in the
coating layer. This is an important requirement of the disclosed
embodiments. Including exfoliated base steel in the coating layer
prevents propagation of cracks formed in the coating layer during
processing. In the case of hot-dip coatings, exfoliation of base
steel causes the interface between the coating layer and the base
steel to become non-smooth. It is believed that this inhibits
propagation of cracks along the interface between the coating layer
and the base steel, thereby improving adhesion. Furthermore, in the
case of alloyed coating layers, for example, in a galvannealed
coating, cracks typically propagate along the interface between the
.GAMMA. phase and the .delta. phase or within the phases, but it is
believed that cracking is inhibited when exfoliated base steel is
included, in other words, when base steel, which is soft, is
present in the alloyed coating layer, which is brittle.
[0039] The amount of exfoliated base steel to be included in the
coating layer is 0.3 to 1.5 g/m.sup.2. If the amount of exfoliated
base steel included is less than 0.3 g/m.sup.2, it is unlikely that
a coating adhesion improvement effect is produced. On the other
hand, if the amount is greater than 1.5 g/m.sup.2, the coating
weight becomes non-uniform, which degrades the appearance.
[0040] Note that "exfoliated base steel" corresponds to the
portions encircled by the solid lines shown in FIG. 1 and are
portions that are exfoliated and completely separated from the base
steel and are incorporated into the coating layer.
[0041] The content of exfoliated base steel can be measured by
using a method described in "Examples" below.
[0042] Next, a method for manufacturing the high-strength
hot-dipped steel sheet having excellent coating adhesion of the
disclosed embodiments will be described.
[0043] A steel slab having a chemical composition as described
above is subjected to hot rolling, rolling at a rolling reduction
ratio of 1 to 10%, and pickling, and subsequently, to rolling at a
rolling reduction ratio of 0.3 to 5% and hot-dip coating. It is
preferable that, in the hot rolling, finish rolling be performed
after rough rolling, at a finish rolling temperature of 820.degree.
C. or higher, and subsequently, coiling be performed at a coiling
temperature of 450 to 650.degree. C. Furthermore, it is preferable
that, after the rolling at a rolling reduction ratio of 0.3 to 5%
and prior to the hot-dip coating, continuous annealing be performed
in a furnace atmosphere having a hydrogen concentration of 2 to 30
vol % and a dew point of -60 to -10.degree. C., with the steel
sheet annealing end-point temperature being 600 to 950.degree. C.
Furthermore, alloying may be additionally performed after the
hot-dip coating. In the disclosed embodiments, performing rolling
prior to and after pickling is an important requirement. Specific
conditions for performing rolling prior to and after pickling will
be described later.
[0044] Hot Rolling
[0045] Hot Rolling Starting Temperature (Slab Heating Temperature)
(Preferable Condition)
[0046] To disperse fine precipitates of Ti, Nb, and the like, it is
necessary to first dissolve Ti, Nb, and the like in the steel sheet
before performing hot rolling. Accordingly, it is preferable that
the heating temperature prior to hot rolling (slab heating
temperature) not be lower than 1100.degree. C. On the other hand,
if the slab is heated to higher than 1300.degree. C., internal
oxidation in a surface layer of the steel may be promoted, and
consequently, surface condition may deteriorate. Accordingly, it is
preferable that the slab heating temperature, prior to hot rolling,
be 1100.degree. C. or higher and 1300.degree. C. or lower. It is
more preferable that the slab heating temperature be 1100.degree.
C. or higher and 1200.degree. C. or lower.
[0047] Finish Rolling Temperature (Preferable Condition)
[0048] It is preferable that the finish rolling temperature not be
lower than 820.degree. C. because deformation resistance for hot
rolling can be reduced, thereby facilitating operation. On the
other hand, if finish rolling is performed at a temperature higher
than 1000.degree. C., scale defects may tend to occur, which may
result in a deterioration of surface condition. Accordingly, the
finish rolling temperature is higher than or equal to 820.degree.
C. and is more preferably 820.degree. C. or higher and 1000.degree.
C. or lower. It is more preferable that the finish rolling
temperature be 850.degree. C. or higher and 950.degree. C. or
lower.
[0049] Hot Rolling Coiling Temperature (Preferable Condition)
[0050] The steel sheet according to the disclosed embodiments
contains oxidizable elements, examples of which include Si, Mn, and
Ti. Accordingly, for inhibiting excessive oxidation of the steel
sheet and ensuring good surface condition, a preferable coiling
temperature is not higher than 650.degree. C. On the other hand, if
the coiling temperature is lower than 450.degree. C., the coil
tends to have poor condition due to uneven cooling, and, therefore,
productivity may decrease. Accordingly, it is preferable that the
hot rolling coiling temperature be 450.degree. C. or higher and
650.degree. C. or lower. It is more preferable that the hot rolling
coiling temperature be 450.degree. C. or higher and 600.degree. C.
or lower.
[0051] Rolling at Rolling Reduction Ratio of 1 to 10% After Hot
Rolling Coiling and Prior to Pickling
[0052] Hot-rolled steel sheets obtained through hot rolling and hot
rolling coiling steps are subjected to pickling for descaling and
thereafter to rolling. The pickling is not particularly limited and
may be performed by using a known method. Here, in the disclosed
embodiments, rolling is performed at a stage prior to pickling.
Performing rolling at a stage prior to pickling is an important
requirement of the disclosed embodiments. The rolling performed at
a stage prior to pickling causes scales to be pressed against the
surface of the steel sheet, thereby imparting suitable
irregularities to the surface to facilitate incorporation of
exfoliated base steel into the coating layer. The rolling reduction
ratio is 1 to 10%. If the rolling reduction ratio is less than 1%,
irregularities are not sufficiently imparted to the surface of the
steel sheet, and it is therefore impossible to produce a sufficient
coating adhesion improvement effect. On the other hand, if the
rolling reduction ratio is greater than 10%, scales bite into the
base steel, which significantly decreases descalability.
[0053] Rolling at Rolling Reduction Ratio of 0.3 to 5% After
Pickling
[0054] In the disclosed embodiments, after pickling, rolling at a
rolling reduction ratio of 0.3 to 5% is performed. Performing
rolling at a rolling reduction ratio of 0.3 to 5% after pickling is
an important requirement of the disclosed embodiments. With the
reduction rolling, a surface topography is controlled, and,
residual stress is introduced into the surface of the base metal.
When the rolling reduction ratio is greater than or equal to 0.3%,
residual stress is sufficiently introduced, and as a result,
reactivity with a coating of the surface of the steel sheet is
improved. If the rolling reduction ratio is greater than 5%, the
effect of improving reactivity with a coating, which is produced by
the introduction of strains, no longer increases, and in addition,
the surface of the steel sheet becomes smooth, which makes
incorporation of exfoliated base steel into the coating layer
difficult.
[0055] Annealing (Preferable Condition)
[0056] It is preferable that annealing be performed after the
rolling at a rolling reduction ratio of 0.3 to 5% and prior to
hot-dip coating. Preferable conditions include a furnace atmosphere
having a hydrogen concentration of 2 to 30 vol % and a dew point of
-60 to -10.degree. C. and a steel sheet annealing end-point
temperature of 600 to 950.degree. C. If the annealing end-point
temperature is a temperature lower than 600.degree. C., the oxide
film after pickling cannot be completely reduced, and consequently,
it may be impossible to obtain desired coating properties.
Furthermore, if the temperature is higher than 950.degree. C., Si,
Mn, and the like are concentrated in the surface, which may degrade
coatability. It is more preferable that the steel sheet annealing
end-point temperature be 650.degree. C. or higher and 850.degree.
C. or lower. It is preferable that the furnace atmosphere have a
hydrogen concentration of 2 to 30% and a dew point of -60 to
-10.degree. C. It is sufficient that the furnace atmosphere be a
reducing atmosphere, and an atmosphere having a dew point of -60 to
-10.degree. C., and a hydrogen concentration of 2 to 30%, the
balance being an inert gas is suitable. If the dew point is higher
than -10.degree. C., Si oxide that is formed in the surface of the
steel sheet tends to be in the form of a film. It is more
preferable that the dew point not be higher than -20.degree. C. On
the other hand, dew points lower than -60.degree. C. are
industrially difficult to realize. If the hydrogen concentration is
lower than 2%, reducibility is too low. When the hydrogen
concentration is lower than or equal to 30%, a sufficient reducing
ability is obtained. It is more preferable that the hydrogen
concentration be 5% or higher and 20% or lower.
[0057] Hot-Dip Coating
[0058] Hot-dip coating is performed in a continuous hot-dip coating
line by using a hot-dip coating bath, preferably after the
annealing of the steel sheet in the reducing atmosphere. For
example, the composition of the hot-dip coating bath is such that,
in the case of hot-dip galvanizing, the Al concentration is within
a range of 0.01 to 0.25% and the balance is Zn and incidental
impurities. If the Al concentration is less than 0.01%, a Zn--Fe
alloying reaction may occur during coating application and a
brittle alloy layer may develop along the interface between the
coating and the steel sheet (base metal), which degrades coating
adhesion. If the Al concentration is greater than 0.25%, an Fe--Al
alloy layer grows noticeably, which impairs coating adhesion. The
temperature of the coating bath does not need to be particularly
limited and may be 440.degree. C. or higher and 480.degree. C. or
lower, which is a usual operation range.
[0059] Alloying (Preferable Condition)
[0060] If the alloying temperature is higher than 550.degree. C., a
.GAMMA. phase, which is hard and brittle, is formed noticeably at
the interface between the steel sheet (base metal) and the coating
film during alloying, and as a result, surface roughness increases
and powdering resistance deteriorates. Accordingly, it is
preferable that the alloying temperature not be higher than
550.degree. C. It is more preferable that the alloying temperature
not be higher than 530.degree. C. On the other hand, if the
alloying temperature is lower than 480.degree. C., sufficient
alloying is not achieved, and it is therefore impossible to obtain
sufficient coating properties. Accordingly, it is desirable that
the alloying temperature not be lower than 480.degree. C.
It is preferable that the alloying time be 10 seconds or more and
60 seconds or less in consideration of problems associated with
cost and control. It is more preferable that the alloying time not
be more than 40 seconds. In the alloying, the method for heating
does not need to be particularly limited, and any of the methods
known in the art, such as radiation heating, conduction heating, or
high-frequency induction heating, may be used. After the alloying
is carried out, the steel sheet is cooled to room temperature.
Treatments after coating application do not need to be particularly
limited, and it is possible to perform temper rolling to adjust the
material, perform leveling or the like to make an adjustment for a
planar shape, and, as necessary, perform a usual aftertreatment,
such as chromating.
EXAMPLES
[0061] Next, the disclosed embodiments will be described in detail
based on examples, but the disclosed embodiments are not limited to
the examples.
Slabs having chemical compositions as shown in Table 1 were used.
Casting was performed using a known method, and subsequently, under
the conditions shown in Table 2, hot rolling, rolling, pickling,
rolling, annealing, and hot-dip galvanizing were performed, and,
for some of the samples, alloying was additionally performed.
[0062] For performing the hot-dip galvanizing, the temperature of
the galvanizing bath was 460.degree. C., and the coating weight was
adjusted to 50 g/m.sup.2 by wiping. The alloying was performed at
an alloying temperature of 520.degree. C.
[0063] Amount of Exfoliated Base Steel in Galvanized Layer
[0064] The amount of exfoliated base steel in the galvanized layer
was measured using the following procedure by ICP emission
spectrometry. First, the galvanized layer of the coated steel sheet
was exclusively removed and dissolved by dissolving the galvanized
layer in dilute hydrochloric acid containing an inhibitor added
thereto. The inhibitor is an additive used to prevent excessive
dissolution of the base steel and may be a commercially available
inhibitor. In the disclosed embodiments, IBIT No. 700BK, which is a
corrosion inhibitor for hydrochloric acid pickling manufactured by
ASAHI Chemical Co., Ltd., was added to hydrochloric acid diluted 10
to 100 fold, such that the concentration of the inhibitor was 0.6
g/L. After dissolution of the coated steel sheet in dilute
hydrochloric acid, a solution including undissolved exfoliated base
steel was extracted and divided into two solutions. One of the
solutions was subjected to composition analysis while the
undissolved exfoliated base steel being with the solution, and the
other of the solutions was subjected to composition analysis after
redissolution by using hydrochloric acid containing no inhibitor.
The amount of exfoliated base steel was determined as the
difference between the results obtained.
[0065] The following tests were conducted on the galvanized steel
sheets obtained as described above to measure a tensile strength
and evaluate the surface appearance of the coating and coating
adhesion. The methods for measurement and the evaluation criteria
are described below.
[0066] Tensile Strength (TS)
[0067] A JIS No. 5 tensile test piece (JIS Z 2201) was cut from a
galvanized steel sheet (GI) or a galvannealed steel sheet (GA) in a
direction perpendicular to the rolling direction. TS was determined
by conducting a tensile test in accordance with the specification
of JIS Z 2241 at a strain rate of 10-.sup.3/s.
[0068] Appearance
[0069] Appearance after hot-dip coating and appearance after
alloying were visually examined, and samples with no bare spots or
alloying unevenness were rated as ".largecircle.", and samples with
bare spots and/or alloying unevenness were rated as "x".
[0070] Coating Adhesion
[0071] The coating adhesion of the galvanized steel sheet was
evaluated by conducting a ball impact test. The ball impact test
was conducted under the conditions including a ball weight of 2.8
kg and a drop height of 1 m. Tape applied to a processed portion
was peeled off, and the presence or absence of peeling of the
coating layer was visually determined. [0072] .largecircle. No
peeling of coating layer occurred [0073] x Peeling of coating layer
occurred
[0074] Powdering Resistance
[0075] The coating adhesion of the galvannealed steel sheet was
evaluated by testing powdering resistance. Cellophane tape was
applied to an alloyed hot-dipped steel sheet, bending at 90 degrees
was applied to the taped surface, which was followed by
bending-back, and then the tape was peeled off. On the basis of
portions of the steel sheet adhering to the peeled tape, the amount
of peeling of the coating per area of 10 mm.times.40 mm of a
bent-back portion was measured by determining the number of Zn
counts by using X-ray fluorescence. The amount was evaluated
according to the following criteria.
TABLE-US-00001 X-ray fluorescence count number Rank Less than 3000:
Excellent 3000 or greater and less than 6000: Good 6000 or greater:
Poor
TABLE-US-00002 TABLE 1 Mass % Steel sample C Si Mn P S Al Ti Nb V
Mo W B Notes A 0.065 0.10 1.60 0.008 0.0008 0.043 0.013 0.02 0.048
0 0 0 Example B 0.070 0.25 1.20 0.005 0.0010 0.005 0.150 0 0 0 0.02
0 Example C 0.100 0.45 2.30 0.010 0.0008 0.046 0.026 0.026 0.042
0.06 0 0 Example D 0.080 0.65 1.45 0.015 0.0010 0.040 0.020 0.010 0
0 0 0.003 Example E 0.070 1.30 1.70 0.005 0.0080 0.005 0.100 0.015
0 0.03 0 0.005 Example F 0.085 2.10 2.12 0.008 0.0008 0.045 0.025
0.025 0.045 0.05 0 0 Comparative example G 0.070 0.70 3.10 0.005
0.0010 0.005 0.150 0 0.150 0 0 0 Comparative example H 0.070 0.25
1.40 0.005 0.0008 0.005 0 0 0.020 0 0.02 0 Example I 0.065 0.15
1.55 0.008 0.0008 0.020 0 0 0 0 0 0 Example
TABLE-US-00003 TABLE 2 Finish Rolling Rolling Steel sheet Tensile
Amount of Steel rolling Coiling reduction reduction Hydrogen Dew
annealing strength exfoliated sample temperature temperature ratio
prior to ratio after concentration point end-point (TS) base steel
Coating Powdering No. used (.degree. C.) (.degree. C.) pickling (%)
pickling (%) (vol %) (.degree. C.) temperature (.degree. C.)
Alloying (MPa) (g/m.sup.2) Appearance adhesion resistance Notes 1 A
850 450 3 0.5 10.1 -35 750 No 720 0.6 Good Good -- Example 2 A 950
600 1 7 8.2 -45 780 Yes 650 0.1 Good -- Poor Comparative example 3
A 920 500 3 1 10.1 -30 620 Yes 785 0.6 Good -- Excellent Example 4
A 920 550 5 3 15.8 -45 880 Yes 608 0.7 Good -- Excellent Example 5
A 950 580 2 0.5 11.5 -25 980 Yes 596 0.5 Good -- Good Example 6 B
940 520 5 1 12.2 -35 580 Yes 602 0.4 Good -- Good Example 7 B 850
420 0.3 4 9.8 -35 800 No 802 0.2 Good Poor -- Comparative example 8
B 880 520 2 1 1.2 -40 650 Yes 650 0.3 Good -- Good Example 9 B 920
500 5 0.5 9.8 -20 700 No 695 0.4 Good Good -- Example 10 B 900 550
3 0.3 7.5 -35 750 Yes 745 0.6 Good -- Excellent Example 11 B 900
600 3 1 12.2 -50 850 Yes 832 0.8 Good -- Excellent Example 12 B 850
500 5 7 9.8 -40 880 Yes 853 0.1 Good -- Poor Comparative example 13
B 950 620 3 0.5 9.8 -40 750 Yes 746 0.9 Good -- Excellent Example
14 B 950 680 4 0.1 12.2 -35 750 Yes 728 1.8 Poor -- Good
Comparative example 15 B 800 480 0.1 4 10.1 -35 850 Yes 805 0.1
Good -- Poor Comparative example 16 C 850 450 0.4 0.5 7.8 -35 620
No 720 0.2 Good Poor -- Comparative example 17 C 940 480 5 0.3 10.1
-35 720 No 789 0.3 Good Good -- Example 18 C 940 550 3 0.5 8.1 -30
670 Yes 750 0.6 Good -- Excellent Example 19 C 980 480 5 0.5 10.1
-45 750 No 805 0.3 Good Good -- Example 20 D 900 610 3 0.5 10.2 -25
720 Yes 765 0.8 Good -- Excellent Example 21 D 850 480 3 1 12.5 -50
780 Yes 802 0.3 Good -- Excellent Example 22 D 900 500 1 10 13.4
-45 800 Yes 835 0.0 Good -- Poor Comparative example 23 D 940 550
13 0.3 12.5 -40 700 Yes 725 1.6 Poor -- Good Comparative example 24
E 850 600 3 1 10.8 -50 700 No 753 0.4 Good Good -- Example 25 E 900
550 2 0.5 10.2 -50 720 No 768 0.5 Good Good -- Example 26 E 940 550
0.3 0.3 12.3 -55 720 Yes 776 0.2 Good -- Poor Comparative example
27 F 920 450 2 1 12.3 -45 750 No 920 0.3 Poor Poor -- Comparative
example 28 F 900 550 3 0.5 15.1 -50 780 No 1050 0.6 Poor Poor --
Comparative example 29 G 940 600 3 0.5 12.2 -40 750 Yes 951 0.8
Good -- Poor Comparative example 30 G 900 550 3 1 10.8 -40 800 Yes
886 0.4 Good -- Poor Comparative example 31 H 880 520 3 1 1.2 -40
650 Yes 670 0.4 Good -- Good Example 32 H 920 500 5 0.5 9.8 -35 700
No 713 0.4 Good Good -- Example 33 A 1050 580 2 1 10.8 -25 850 Yes
629 0.5 Good -- Good Example 34 E 900 550 2 0.5 10.2 -5 800 Yes 830
0.6 Good -- Good Example 35 I 920 500 3 1 10.1 -30 650 Yes 760 0.6
Good -- Excellent Example
[0076] Table 2 demonstrates that Examples have good surface
appearance and good coating adhesion (powdering resistance). In
contrast, Comparative Examples are unsatisfactory in terms of at
least one of surface appearance and coating adhesion (powdering
resistance).
INDUSTRIAL APPLICABILITY
[0077] The high-strength hot-dipped steel sheet of the disclosed
embodiments suitable for use in automotive parts, for which, in
recent years, designing for increased strength and reduced
thickness has been rapidly advanced.
* * * * *